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<div><h2 class="title">
<a name="base_from_member"></a>Base_From_Member</h2></div>
<div><div class="authorgroup"><div class="author"><h3 class="author">
<span class="firstname">Daryle</span> <span class="surname">Walker</span>
</h3></div></div></div>
<div><p class="copyright">Copyright © 2001, 2003, 2004, 2012 Daryle
      Walker</p></div>
<div><div class="legalnotice">
<a name="base_from_member.legal"></a><p>
        Distributed under the Boost Software License, Version 1.0. (See accompanying
        file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
      </p>
</div></div>
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</div>
<div class="toc">
<p><b>Table of Contents</b></p>
<dl class="toc">
<dt><span class="section"><a href="base_from_member.html#base_from_member.rationale">Rationale</a></span></dt>
<dt><span class="section"><a href="base_from_member.html#base_from_member.synopsis">Synopsis</a></span></dt>
<dt><span class="section"><a href="base_from_member.html#base_from_member.usage">Usage</a></span></dt>
<dt><span class="section"><a href="base_from_member.html#base_from_member.example">Example</a></span></dt>
<dt><span class="section"><a href="base_from_member.html#base_from_member.acknowledgments">Acknowledgments</a></span></dt>
</dl>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="base_from_member.rationale"></a><a class="link" href="base_from_member.html#base_from_member.rationale" title="Rationale">Rationale</a>
</h2></div></div></div>
<p>
      When developing a class, sometimes a base class needs to be initialized with
      a member of the current class. As a naïve example:
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">streambuf</span><span class="special">&gt;</span>  <span class="comment">/* for std::streambuf */</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">ostream</span><span class="special">&gt;</span>    <span class="comment">/* for std::ostream */</span>

<span class="keyword">class</span> <span class="identifier">fdoutbuf</span>
  <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">streambuf</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdoutbuf</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">fdostream</span>
  <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span>
<span class="special">{</span>
<span class="keyword">protected</span><span class="special">:</span>
    <span class="identifier">fdoutbuf</span> <span class="identifier">buf</span><span class="special">;</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdostream</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">buf</span><span class="special">(</span> <span class="identifier">fd</span> <span class="special">),</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">buf</span> <span class="special">)</span> <span class="special">{}</span>
    <span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
      This is undefined because C++'s initialization order mandates that the base
      class is initialized before the member it uses. <a href="http://www.moocat.org" target="_top">R.
      Samuel Klatchko</a> developed a way around this by using the initialization
      order in his favor. Base classes are intialized in order of declaration, so
      moving the desired member to another base class, that is initialized before
      the desired base class, can ensure proper initialization.
    </p>
<p>
      A custom base class can be made for this idiom:
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">streambuf</span><span class="special">&gt;</span>  <span class="comment">/* for std::streambuf */</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">ostream</span><span class="special">&gt;</span>    <span class="comment">/* for std::ostream */</span>

<span class="keyword">class</span> <span class="identifier">fdoutbuf</span>
  <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">streambuf</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdoutbuf</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">struct</span> <span class="identifier">fdostream_pbase</span>
<span class="special">{</span>
    <span class="identifier">fdoutbuf</span> <span class="identifier">sbuffer</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="identifier">fdostream_pbase</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">sbuffer</span><span class="special">(</span> <span class="identifier">fd</span> <span class="special">)</span> <span class="special">{}</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">fdostream</span>
  <span class="special">:</span> <span class="keyword">private</span> <span class="identifier">fdostream_pbase</span>
  <span class="special">,</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span>
<span class="special">{</span>
    <span class="keyword">typedef</span> <span class="identifier">fdostream_pbase</span>  <span class="identifier">pbase_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span>     <span class="identifier">base_type</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdostream</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">pbase_type</span><span class="special">(</span> <span class="identifier">fd</span> <span class="special">),</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">sbuffer</span> <span class="special">)</span> <span class="special">{}</span>
    <span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
      Other projects can use similar custom base classes. The technique is basic
      enough to make a template, with a sample template class in this library. The
      main template parameter is the type of the enclosed member. The template class
      has several (explicit) constructor member templates, which implicitly type
      the constructor arguments and pass them to the member. The template class uses
      implicit copy construction and assignment, cancelling them if the enclosed
      member is non-copyable.
    </p>
<p>
      Manually coding a base class may be better if the construction and/or copying
      needs are too complex for the supplied template class, or if the compiler is
      not advanced enough to use it.
    </p>
<p>
      Since base classes are unnamed, a class cannot have multiple (direct) base
      classes of the same type. The supplied template class has an extra template
      parameter, an integer, that exists solely to provide type differentiation.
      This parameter has a default value so a single use of a particular member type
      does not need to concern itself with the integer.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="base_from_member.synopsis"></a><a class="link" href="base_from_member.html#base_from_member.synopsis" title="Synopsis">Synopsis</a>
</h2></div></div></div>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">type_traits</span><span class="special">&gt;</span>  <span class="comment">/* exposition only */</span>

<span class="preprocessor">#ifndef</span> <span class="identifier">BOOST_BASE_FROM_MEMBER_MAX_ARITY</span>
<span class="preprocessor">#define</span> <span class="identifier">BOOST_BASE_FROM_MEMBER_MAX_ARITY</span>  <span class="number">10</span>
<span class="preprocessor">#endif</span>

<span class="keyword">template</span> <span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">MemberType</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">UniqueID</span> <span class="special">=</span> <span class="number">0</span> <span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span>
<span class="special">{</span>
<span class="keyword">protected</span><span class="special">:</span>
    <span class="identifier">MemberType</span>  <span class="identifier">member</span><span class="special">;</span>

<span class="preprocessor">#if</span> <span class="emphasis"><em>C++11 is in use</em></span>
    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="special">...</span><span class="identifier">T</span> <span class="special">&gt;</span>
    <span class="keyword">explicit</span> <span class="keyword">constexpr</span>   <span class="identifier">base_from_member</span><span class="special">(</span> <span class="identifier">T</span><span class="special">&amp;&amp;</span> <span class="special">...</span><span class="identifier">x</span> <span class="special">)</span>
     <span class="keyword">noexcept</span><span class="special">(</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">is_nothrow_constructible</span><span class="special">&lt;</span><span class="identifier">MemberType</span><span class="special">,</span> <span class="identifier">T</span><span class="special">...&gt;::</span><span class="identifier">value</span> <span class="special">);</span>
<span class="preprocessor">#else</span>
    <span class="identifier">base_from_member</span><span class="special">();</span>

    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T1</span> <span class="special">&gt;</span>
    <span class="keyword">explicit</span>  <span class="identifier">base_from_member</span><span class="special">(</span> <span class="identifier">T1</span> <span class="identifier">x1</span> <span class="special">);</span>

    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span> <span class="special">&gt;</span>
    <span class="identifier">base_from_member</span><span class="special">(</span> <span class="identifier">T1</span> <span class="identifier">x1</span><span class="special">,</span> <span class="identifier">T2</span> <span class="identifier">x2</span> <span class="special">);</span>

    <span class="comment">//...</span>

    <span class="keyword">template</span><span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">T1</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T2</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T3</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T4</span><span class="special">,</span>
     <span class="keyword">typename</span> <span class="identifier">T5</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T6</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T7</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T8</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">T9</span><span class="special">,</span>
     <span class="keyword">typename</span> <span class="identifier">T10</span> <span class="special">&gt;</span>
    <span class="identifier">base_from_member</span><span class="special">(</span> <span class="identifier">T1</span> <span class="identifier">x1</span><span class="special">,</span> <span class="identifier">T2</span> <span class="identifier">x2</span><span class="special">,</span> <span class="identifier">T3</span> <span class="identifier">x3</span><span class="special">,</span> <span class="identifier">T4</span> <span class="identifier">x4</span><span class="special">,</span> <span class="identifier">T5</span> <span class="identifier">x5</span><span class="special">,</span> <span class="identifier">T6</span> <span class="identifier">x6</span><span class="special">,</span> <span class="identifier">T7</span> <span class="identifier">x7</span><span class="special">,</span>
     <span class="identifier">T8</span> <span class="identifier">x8</span><span class="special">,</span> <span class="identifier">T9</span> <span class="identifier">x9</span><span class="special">,</span> <span class="identifier">T10</span> <span class="identifier">x10</span> <span class="special">);</span>
<span class="preprocessor">#endif</span>
<span class="special">};</span>

<span class="keyword">template</span> <span class="special">&lt;</span> <span class="keyword">typename</span> <span class="identifier">MemberType</span><span class="special">,</span> <span class="keyword">int</span> <span class="identifier">UniqueID</span> <span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">MemberType</span><span class="special">&amp;,</span> <span class="identifier">UniqueID</span><span class="special">&gt;</span>
<span class="special">{</span>
<span class="keyword">protected</span><span class="special">:</span>
    <span class="identifier">MemberType</span><span class="special">&amp;</span> <span class="identifier">member</span><span class="special">;</span>

    <span class="keyword">explicit</span> <span class="keyword">constexpr</span> <span class="identifier">base_from_member</span><span class="special">(</span> <span class="identifier">MemberType</span><span class="special">&amp;</span> <span class="identifier">x</span> <span class="special">)</span>
        <span class="keyword">noexcept</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
      The class template has a first template parameter <code class="computeroutput"><span class="identifier">MemberType</span></code>
      representing the type of the based-member. It has a last template parameter
      <code class="computeroutput"><span class="identifier">UniqueID</span></code>, that is an <code class="computeroutput"><span class="keyword">int</span></code>, to differentiate between multiple base
      classes that use the same based-member type. The last template parameter has
      a default value of zero if it is omitted. The class template has a protected
      data member called <code class="computeroutput"><span class="identifier">member</span></code> that
      the derived class can use for later base classes (or itself).
    </p>
<p>
      If the appropriate features of C++11 are present, there will be a single constructor
      template. It implements <span class="emphasis"><em>perfect forwarding</em></span> to the best
      constructor call of <code class="computeroutput"><span class="identifier">member</span></code>
      (if any). The constructor template is marked both <code class="computeroutput"><span class="keyword">constexpr</span></code>
      and <code class="computeroutput"><span class="keyword">explicit</span></code>. The former will
      be ignored if the corresponding inner constructor call (of <code class="computeroutput"><span class="identifier">member</span></code>)
      does not have the marker. The latter binds the other way; always taking effect,
      even when the inner constructor call does not have the marker. The constructor
      template propagates the <code class="computeroutput"><span class="keyword">noexcept</span></code>
      status of the inner constructor call. (The constructor template has a trailing
      parameter with a default value that disables the template when its signature
      is too close to the signatures of the automatically-defined non-template copy-
      and/or move-constructors of <code class="computeroutput"><span class="identifier">base_from_member</span></code>.)
    </p>
<p>
      On earlier-standard compilers, there is a default constructor and several constructor
      member templates. These constructor templates can take as many arguments (currently
      up to ten) as possible and pass them to a constructor of the data member.
    </p>
<p>
      A specialization for member references offers a single constructor taking a
      <code class="computeroutput"><span class="identifier">MemberType</span><span class="special">&amp;</span></code>,
      which is the only way to initialize a reference.
    </p>
<p>
      Since C++ does not allow any way to explicitly state the template parameters
      of a templated constructor, make sure that the arguments are already close
      as possible to the actual type used in the data member's desired constructor.
      Explicit conversions may be necessary.
    </p>
<p>
      The <code class="computeroutput"><span class="identifier">BOOST_BASE_FROM_MEMBER_MAX_ARITY</span></code>
      macro constant specifies the maximum argument length for the constructor templates.
      The constant may be overridden if more (or less) argument configurations are
      needed. The constant may be read for code that is expandable like the class
      template and needs to maintain the same maximum size. (Example code would be
      a class that uses this class template as a base class for a member with a flexible
      set of constructors.) This constant is ignored when C++11 features are present.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="base_from_member.usage"></a><a class="link" href="base_from_member.html#base_from_member.usage" title="Usage">Usage</a>
</h2></div></div></div>
<p>
      With the starting example, the <code class="computeroutput"><span class="identifier">fdoutbuf</span></code>
      sub-object needs to be encapsulated in a base class that is inheirited before
      <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>.
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">base_from_member</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">streambuf</span><span class="special">&gt;</span>  <span class="comment">// for std::streambuf</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">ostream</span><span class="special">&gt;</span>    <span class="comment">// for std::ostream</span>

<span class="keyword">class</span> <span class="identifier">fdoutbuf</span>
  <span class="special">:</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">streambuf</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdoutbuf</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">fdostream</span>
  <span class="special">:</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">fdoutbuf</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">public</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span>
<span class="special">{</span>
    <span class="comment">// Helper typedef's</span>
    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">fdoutbuf</span><span class="special">&gt;</span>  <span class="identifier">pbase_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span>                        <span class="identifier">base_type</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fdostream</span><span class="special">(</span> <span class="keyword">int</span> <span class="identifier">fd</span> <span class="special">)</span>
      <span class="special">:</span> <span class="identifier">pbase_type</span><span class="special">(</span> <span class="identifier">fd</span> <span class="special">),</span> <span class="identifier">base_type</span><span class="special">(</span> <span class="special">&amp;</span><span class="identifier">member</span> <span class="special">){}</span>
    <span class="comment">//...</span>
<span class="special">};</span>
</pre>
<p>
      The base-from-member idiom is an implementation detail, so it should not be
      visible to the clients (or any derived classes) of <code class="computeroutput"><span class="identifier">fdostream</span></code>.
      Due to the initialization order, the <code class="computeroutput"><span class="identifier">fdoutbuf</span></code>
      sub-object will get initialized before the <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">ostream</span></code>
      sub-object does, making the former sub-object safe to use in the latter sub-object's
      construction. Since the <code class="computeroutput"><span class="identifier">fdoutbuf</span></code>
      sub-object of the final type is the only sub-object with the name <code class="computeroutput"><span class="identifier">member</span></code> that name can be used unqualified
      within the final class.
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="base_from_member.example"></a><a class="link" href="base_from_member.html#base_from_member.example" title="Example">Example</a>
</h2></div></div></div>
<p>
      The base-from-member class templates should commonly involve only one base-from-member
      sub-object, usually for attaching a stream-buffer to an I/O stream. The next
      example demonstrates how to use multiple base-from-member sub-objects and the
      resulting qualification issues.
    </p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">base_from_member</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstddef</span><span class="special">&gt;</span>  <span class="comment">/* for NULL */</span>

<span class="keyword">struct</span> <span class="identifier">an_int</span>
<span class="special">{</span>
    <span class="keyword">int</span>  <span class="identifier">y</span><span class="special">;</span>

    <span class="identifier">an_int</span><span class="special">(</span> <span class="keyword">float</span> <span class="identifier">yf</span> <span class="special">);</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">switcher</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">switcher</span><span class="special">();</span>
    <span class="identifier">switcher</span><span class="special">(</span> <span class="keyword">double</span><span class="special">,</span> <span class="keyword">int</span> <span class="special">*</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">flow_regulator</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">flow_regulator</span><span class="special">(</span> <span class="identifier">switcher</span> <span class="special">&amp;,</span> <span class="identifier">switcher</span> <span class="special">&amp;</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">template</span> <span class="special">&lt;</span> <span class="keyword">unsigned</span> <span class="identifier">Size</span> <span class="special">&gt;</span>
<span class="keyword">class</span> <span class="identifier">fan</span>
<span class="special">{</span>
<span class="keyword">public</span><span class="special">:</span>
    <span class="keyword">explicit</span> <span class="identifier">fan</span><span class="special">(</span> <span class="identifier">switcher</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="keyword">class</span> <span class="identifier">system</span>
  <span class="special">:</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">an_int</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">private</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>
  <span class="special">,</span> <span class="keyword">protected</span> <span class="identifier">flow_regulator</span>
  <span class="special">,</span> <span class="keyword">public</span> <span class="identifier">fan</span><span class="special">&lt;</span><span class="number">6</span><span class="special">&gt;</span>
<span class="special">{</span>
    <span class="comment">// Helper typedef's</span>
    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">an_int</span><span class="special">&gt;</span>       <span class="identifier">pbase0_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">&gt;</span>     <span class="identifier">pbase1_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">,</span> <span class="number">1</span><span class="special">&gt;</span>  <span class="identifier">pbase2_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span><span class="identifier">switcher</span><span class="special">,</span> <span class="number">2</span><span class="special">&gt;</span>  <span class="identifier">pbase3_type</span><span class="special">;</span>

    <span class="keyword">typedef</span> <span class="identifier">flow_regulator</span>  <span class="identifier">base1_type</span><span class="special">;</span>
    <span class="keyword">typedef</span> <span class="identifier">fan</span><span class="special">&lt;</span><span class="number">6</span><span class="special">&gt;</span>          <span class="identifier">base2_type</span><span class="special">;</span>

<span class="keyword">public</span><span class="special">:</span>
    <span class="identifier">system</span><span class="special">(</span> <span class="keyword">double</span> <span class="identifier">x</span> <span class="special">);</span>
    <span class="comment">//...</span>
<span class="special">};</span>

<span class="identifier">system</span><span class="special">::</span><span class="identifier">system</span><span class="special">(</span> <span class="keyword">double</span> <span class="identifier">x</span> <span class="special">)</span>
  <span class="special">:</span> <span class="identifier">pbase0_type</span><span class="special">(</span> <span class="number">0.2</span> <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">pbase1_type</span><span class="special">()</span>
  <span class="special">,</span> <span class="identifier">pbase2_type</span><span class="special">(</span> <span class="special">-</span><span class="number">16</span><span class="special">,</span> <span class="special">&amp;</span><span class="keyword">this</span><span class="special">-&gt;</span><span class="identifier">pbase0_type</span><span class="special">::</span><span class="identifier">member</span><span class="special">.</span><span class="identifier">y</span> <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">pbase3_type</span><span class="special">(</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">static_cast</span><span class="special">&lt;</span><span class="keyword">int</span> <span class="special">*&gt;(</span><span class="identifier">NULL</span><span class="special">)</span> <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">base1_type</span><span class="special">(</span> <span class="identifier">pbase3_type</span><span class="special">::</span><span class="identifier">member</span><span class="special">,</span> <span class="identifier">pbase1_type</span><span class="special">::</span><span class="identifier">member</span> <span class="special">)</span>
  <span class="special">,</span> <span class="identifier">base2_type</span><span class="special">(</span> <span class="identifier">pbase2_type</span><span class="special">::</span><span class="identifier">member</span> <span class="special">)</span>
<span class="special">{</span>
    <span class="comment">//...</span>
<span class="special">}</span>
</pre>
<p>
      The final class has multiple sub-objects with the name <code class="computeroutput"><span class="identifier">member</span></code>,
      so any use of that name needs qualification by a name of the appropriate base
      type. (Using <code class="computeroutput"><span class="keyword">typedef</span></code>s ease mentioning
      the base types.) However, the fix introduces a new problem when a pointer is
      needed. Using the address operator with a sub-object qualified with its class's
      name results in a pointer-to-member (here, having a type of <code class="computeroutput"><span class="identifier">an_int</span>
      <span class="identifier">boost</span><span class="special">::</span><span class="identifier">base_from_member</span><span class="special">&lt;</span>
      <span class="identifier">an_int</span><span class="special">,</span>
      <span class="number">0</span><span class="special">&gt;</span> <span class="special">::</span> <span class="special">*</span></code>) instead
      of a pointer to the member (having a type of <code class="computeroutput"><span class="identifier">an_int</span>
      <span class="special">*</span></code>). The new problem is fixed by qualifying
      the sub-object with <code class="computeroutput"><span class="keyword">this</span><span class="special">-&gt;</span></code>
      and is needed just for pointers, and not for references or values.
    </p>
<p>
      There are some argument conversions in the initialization. The constructor
      argument for <code class="computeroutput"><span class="identifier">pbase0_type</span></code> is
      converted from <code class="computeroutput"><span class="keyword">double</span></code> to <code class="computeroutput"><span class="keyword">float</span></code>. The first constructor argument for <code class="computeroutput"><span class="identifier">pbase2_type</span></code> is converted from <code class="computeroutput"><span class="keyword">int</span></code> to <code class="computeroutput"><span class="keyword">double</span></code>.
      The second constructor argument for <code class="computeroutput"><span class="identifier">pbase3_type</span></code>
      is a special case of necessary conversion; all forms of the null-pointer literal
      in C++ (except <code class="computeroutput"><span class="keyword">nullptr</span></code> from C++11)
      also look like compile-time integral expressions, so C++ always interprets
      such code as an integer when it has overloads that can take either an integer
      or a pointer. The last conversion is necessary for the compiler to call a constructor
      form with the exact pointer type used in <code class="computeroutput"><span class="identifier">switcher</span></code>'s
      constructor. (If C++11's <code class="computeroutput"><span class="keyword">nullptr</span></code>
      is used, it still needs a conversion if multiple pointer types can be accepted
      in a constructor call but <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">nullptr_t</span></code>
      cannot.)
    </p>
</div>
<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="base_from_member.acknowledgments"></a><a class="link" href="base_from_member.html#base_from_member.acknowledgments" title="Acknowledgments">Acknowledgments</a>
</h2></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
          <a href="http://www.boost.org/people/ed_brey.htm" target="_top">Ed Brey</a> suggested
          some interface changes.
        </li>
<li class="listitem">
          <a href="http://www.moocat.org" target="_top">R. Samuel Klatchko</a> (<a href="mailto:rsk%40moocat.org" target="_top">rsk@moocat.org</a>,
          <a href="mailto:rsk%40brightmail.com" target="_top">rsk@brightmail.com</a>) invented
          the idiom of how to use a class member for initializing a base class.
        </li>
<li class="listitem">
          <a href="http://www.boost.org/people/dietmar_kuehl.htm" target="_top">Dietmar Kuehl</a>
          popularized the base-from-member idiom in his <a href="http://www.informatik.uni-konstanz.de/~kuehl/c++/iostream/" target="_top">IOStream
          example classes</a>.
        </li>
<li class="listitem">
          Jonathan Turkanis supplied an implementation of generating the constructor
          templates that can be controlled and automated with macros. The implementation
          uses the <a href="../../../preprocessor/index.html" target="_top">Preprocessor library</a>.
        </li>
<li class="listitem">
          <a href="http://www.boost.org/people/daryle_walker.html%22%3eDaryle" target="_top">Walker</a>
          started the library. Contributed the test file <a href="../../test/base_from_member_test.cpp" target="_top">base_from_member_test.cpp</a>.
        </li>
</ul></div>
</div>
</div>
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<td align="left"><p><small>Last revised: August 05, 2021 at 09:45:10 GMT</small></p></td>
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